A reinterpretation of the Wagendrift Quarry

ROBERT W. SELOVER AND ROBERT A. GASTALDO

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A reinterpretation of the Wagendrift Quarry, Estcourt, KwaZulu-Natal Province, and its implications for Karoo Basin Paleogeography Robert W. Selover and Robert A. Gastaldo Department of Geology, Colby College Waterville, ME 04901 USA e-mail: [email protected]; [email protected] © 2005 Geological Society of South Africa

ABSTRACT The South African Karoo Basin preserves a continental record across the Permian/Triassic Boundary in which both plant and vertebrate fossil assemblages may co-occur. The Upper Permian part of the Beaufort Group, a shallow-water fluvial succession spans this boundary. The Estcourt Formation previously was interpreted to be part of this group representing shallow bay fill, the result of overbank deposition. This interpretation implies that sedimentation occurred along the margins of a large, shallowing basin during the Late Permian. Recent work in a series of outcrops at Wagendrift Dam, near the town of Estcourt, requires a reinterpretation of this setting. Three outcrops consist of eight facies, six of which are characteristic of a submarine turbidite system. Basal-most massively bedded sandstones, interpreted as submarine channel deposits are overlain by millimeter-scale, fining upwards couplets of siltstone to mudstone that are distal turbidite in origin. Millimeter-scale bedding is undisturbed and bioturbation is limited, indicating deposition within a low oxygen zone. Ripple cross laminated and ball-and-pillow sandstones, overlain by a finer, massive siltstone are indicative of more proximal turbidites, which is further supported by the presence of large, localized olistoliths (slump blocks) in which internal bedding is preserved. Millimeter-to-centimeter scale upwards-fining successions of planar to ripple cross laminated beds of coarser (fine sand, coarse silt) sedimentary rocks overlain by finer siltstone represent a continuation of proximal turbidite conditions. Thick, ripple-bedded sandstones cap the section, representing a major change in depositional setting into a turbidite fan channel system. The sedimentological characteristics of this succession suggest that although these rocks are assigned to the Latest Permian Beaufort Group, they are, in fact, part of the Ecca Group. Palynomorphs recovered from the quarry are of Late Permian age and consist of taeniate and non-taeniate bisaccate pollen, and spores. Assignment of the assemblage, based on partial canonical correspondence analysis, places the Wagendrift palynoflora in the lower part of Aitken’s Biozone VI (Volkrust Formation; Wujiapingian). These results indicate that turbidite sedimentation similar to that in the southern and western part of the basin persisted into the Late Permian of KwaZulu-Natal.

Introduction The rocks that comprise the Karoo Supergroup of South Africa record sedimentation from the Late Carboniferous to the Early Jurassic (Dwyka, Ecca, Beaufort and “Stromberg” groups), and generation of the igneous Drankenberg Group (Smith et al., 1993; Johnson et al., 1997). Sedimentation began following the retreat of Late Carboniferous-Middle Permian glaciers towards the southeastern Cape Fold Belt highlands that resulted in a deep-water environment (Smith, 1990). The subsequent shallowing of this basin witnessed emplacement of fluvio-lacustrine and desert deposits (Johnson et al., 1997) within the retroarc foreland basin (Catuneanu et al., 1998). Karoo sedimentation began in the CarboniferousPermian with deposition of tillites, sandstones, and mudstones of the Dwyka Group, which lie unconformably atop Paleozoic and Precambrian bedrock (Smith, 1990). Dwyka sedimentation is linked to multidirectional ice-lobe orientations, ranging from north to northeast, east and south, during glacial retreat (Smith, 1990 and references therein). As these glaciers disappeared, sedimentation shifted from high to lower energy depositional regimes in which sediments

accumulated in a deep lake or landlocked sea (Cairncross et al., 2004). This basin was filled gradually by Permian deposits that represent a primarily prograding sediment shelf, which is marked by Boumastyle turbidite deposits, and are assigned to the Ecca Group (Smith, 1990; Johnson et al., 1997). These turbidite-dominated deposits extend from the south and southwestern parts of the basin (Collingham, Vischkuil, and Ripon Formations) to the west and north (Laingsburg Formation.; Johnson et al., 1997). These are overlain by prodelta, shallow shelf, and deltaic deposits. Towards the east, the Ecca facies grade into primarily deltaic and lacustrine deposits with a shift to shallow water, meandering fluvial channels and floodplain lacustrine regimes characteristic of the Permian-Triassic Beaufort Group (Smith, 1990; Johnson et al., 1997). Fluvial and lacustrine depositional environments persisted throughout the Triassic into the Early Jurassic (Molteno and Elliot Formations) until facies reflect a change to eolian deposits with dune fields and playas (Clarens Formation). Sedimentation ended with Gondwanan rifting and formation of the Middle Jurassic Drakensberg intrusive and extrusive rocks (Smith, 1990; Duncan et al., 1997).

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A REINTERPRETATION OF THE WAGENDRIFT QUARRY, KWAZULU-NATAL and most laterally extensive outcrop is a ‘quarry’ exploited for road material and first described by Van Dijk et al. (1978). It is exposed at the intersection of two roads across from the Wendover Farm, and is 116 m in length, 7 m high with a strike of 193° and a dip of 2° to the northeast. The second outcrop is immediately (~100 m) to the southeast of the quarry along a small private roadcut. It is approximately 50 m in length and 6 m high. The last exposure is within a series of dongas exposed on the Wendover Farm. The donga section was examined but not measured due to limited exposure and the lack of stratigraphic control because of tectonic disruption as the result of Jurassic dolerite intrusions. In general, the dolerite intrusions hamper the regionalscale correlation of the studied units. The project focus was on the quarry and road-cut outcrops because each contained laterally extensive, continuous beds oriented perpendicular to each other providing for some threedimensional control.

Figure 1. Generalized locality map of collection sites near Wagendrift Dam in Estcourt, KwaZulu-Natal Province (blackened). Three outcrops are exposed in this area within a quarry, a roadcut, and a donga. All three localities are near the Wendover Farm, as marked on the 1:50,000 map (2929BB) of Estcourt Quadrangle.

The Estcourt Formation is part of the Beaufort Group and is restricted to KwaZulu-Natal Province. It consists of about 400 m of carbonaceous mudstone and sandstone with sparse, intermittently distributed coals (Johnson et al., 1997). It is mapped currently as the edge of Lower Beaufort deposits in the basin. Van Dijk et al. (1978) interpreted the exposure at the Wagendrift Dam section as bayfill overbank deposits representing the shallowing of internal deep-water settings. This interpretation places Wagendrift and, hence, the Estcourt Formation within the Beaufort Group despite a diachronous Ecca/Beaufort transition from the south to the north within the basin (Modesto et al., 2001). This temporal relationship suggests that there may be some variability in the stratigraphic assignments of these rocks, which is further supported by a reevaluation of the Wagendrift section. This study was undertaken in an attempt to evaluate the previously interpreted depositional environment as part of an overall broader project investigating terrestrial diversity trends across the Permian/Triassic boundary (Sims et al., 2004; Gastaldo et al., in press). Locality Description The Estcourt Formation is located in the Mooi River/Bergville/Ladysmith area (Figure 1) in KwaZuluNatal Province of South Africa (Johnson et al., 1997). Several outcrops occur to the southwest of the town of Estcourt, near the Beechwood campgrounds of the Wagendrift Nature Reserve. The section consists of 3 outcrops in close proximity to each other. The primary

Lithological Description The Wagendrift section consists of massive beds of fine sandstone, couplets of siltstone/mudstone and sandstone/siltstone, along with several interbedded sandstones exhibiting various primary sedimentary structures (e.g., ripples, balls-and-pillows). The section also includes a large, laterally extensive bed of diagenetically altered siltstone, and a thin, homogenous bed of fine grained, microcrystalline tuff. The section is subdivided into eight lithofacies based on sediment characteristics and cyclical depositional patterns (Figure 2). Facies are laterally extensive, consistent between outcrops, and several key beds can be traced from exposure to exposure. The true thickness of the studied sedimentary unit is unknown, and its base was taken at the top of the local dolerite sill (Figures 2; 3). Facies 1. Yellowish Gray Very Fine Sandstone. The basalmost lithotype consists of four beds of yellowish gray (5Y 7/2) very fine sandstone, with beds ranging in thickness from 32 to 75 cm, in direct contact with the underlying dolerite sill (Figure 3). This facies is found only in the road cut. One sandstone bed contains matrix supported intraformational mud clasts, whereas the others are massive. No other primary sedimentary structures of any scale were observed. In thin section, this lithology consists of subangular to subrounded, very fine (mean = 0.08 mm) quartz (98% Qtz, N=300) clasts. Facies 2. Dark to Dusky Yellowish Brown Very Fine Sandy Siltstone This unit overlies the basal sandstone and consists of a fining-upwards succession of undetermined thickness (minimum 60-80 cm) of dark to dusky yellowish brown (10 YR 4/2-2/2, weathered dark yellowish orange to moderate yellowish brown, 10 YR 6/6- 5/4) very fine sandy siltstone (Figure 4). The minimum thickness estimate is due to the exposure beginning at the base of the quarry. There are no primary structures and the unit

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Figure 3. Basal sandstone as exposed at the roadcut section where it is in direct contact with the underlying dolerite. The contact is irregular, and the altered zone in the sedimentary rocks appeared to be minor in the field (thin section analysis was inconclusive).

Figure 2. Stratigraphic column with schematic line drawing. The basal sandstone with intraformational mudclasts characterizing facies 1 (F1) represents turbidite channel deposits. The chaotic deposits laterally adjacent to olistolith slump blocks characterize facies 2 (F2). Facies 3 (F3) represents distal turbidite deposition. Facies 4, 6 and 7 (F4, F6, F7) represent more proximal turbidite deposits as expressed by fining upward successions with planar, rippled, and ball-and-pillowed sandstone beds at the base. Facies 5 (F5) is a thin ash bed. The rippled sandstones of facies 8 (F8) represent a more proximal position relative to the turbidite channel.

has been extensively weathered, such that recovery of a sample for thin section analysis was precluded. The extensively weathered unit is laterally adjacent to three olistoliths (slump blocks, Figure 4) at the northwest end of the quarry section. The contact with the overlying facies is sharp. Facies 3. Light Brown Coarse Siltstone/Yellow to Pale Olive Mudstone Successions Conformably overlying Facies 2 is a thick (102 cm) unit consisting of couplets of upward-fining successions of mm-to-cm scale planar/ripple cross laminated beds of light brown (5YR 6/4) coarse siltstone to very fine

sandstone overlain by yellow to pale olive (5Y 6/4 to 10Y 6/2) siltstone to mudstone (Figure 4). The basal sandstone lamina of each couplet becomes thicker and more prominent upsection with a minimum thickness of 0.1 cm and a maximum of 1.2 cm. Sandstone beds in the lower part are mm-scale laminations, whereas they become thicker (cm-scale) and more pronounced upsection. The average thickness of sandstone beds is 0.3 cm (N=29). This lithofacies is identical to that found within the slump blocks in the northwest corner of the quarry (Figure 4). In cross section, these blocks are convex at their upper surface and lie laterally adjacent to each other. Blocks decrease in size from northwest to southeast. No block is exposed completely, but each has an estimated thickness of at least 15 cm and is 30 cm in length. The upper contact of the blocks with the overlying conformable couplets, which onlap each block, is weathered allowing for easy recognition. Rare horizontal and vertical burrows are scattered in this facies along with rare plant fossils that are preserved as impressions parallel to bedding. The cylindrical vertical burrows average 0.3 cm in diameter and are 2.5 to 3.0 cm long, while the elliptical horizontal burrows average 0.6 cm in diameter with an undetermined length. One Fugichnia (escape structure) was observed. Small Glossopteris leaves and a whorl of Phyllotheca cf. australis (Lacey et al., 1975) are found isolated and dispersed in the basalmost siltstone/mudstone couplets (Figures 5A to C). Plant fossils were not recovered from this facies higher in the section although Cindy Looy (personal communication, 2003) has recovered pollen (Figures 5D; G). Paleocurrent data are based on the rippled sandstone beds and distributed bimodally. They show flow directions either from the northeast [mean vector = 50°, N=21] or southwest [mean vector = 228°, N=10] (Figure 6). Thin section analysis revealed thick laminae (5 to 10 mm), with fining-upward successions of very fine sandstone (mean d=0.10 mm) to medium siltstone (mean d=0.03 mm), separated by gradational to abrupt-


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A REINTERPRETATION OF THE WAGENDRIFT QUARRY, KWAZULU-NATAL

Figure 4. Olistolith and chaotic deposit overlain by laminated siltstone (A) with line drawing to scale (B). The olistolith and chaotic deposit are heavily weathered, and often rubbly, but are easily distinguished in the field. This initial short-lived event is overlain by sediments of more periodic depositional events. Scale = 0.5 m.

contacts. The grain mineralogy consists of 98% quartz (N=300), with the remaining 2% as small (0.4 mm), irregular, platy fragments of an isotropic mineral. Facies 4. Diagenetic Nodule-Rich Olive Gray Siltstone Conformably overlying Facies 3 is a planar/ripple cross laminated bed of very pale to grayish orange (10YR 8/27/4) coarse siltstone to very fine sandstone, overlain by 15 to 20 cm of a homogenous, massive bed of olive gray (5Y 3/2) coarse siltstone (Figure 2). The primary structures in this interval are similar to Facies 3 but have been altered diagenetically, as evidenced by their rustcolor and friability. This interval contains grayish red to dark reddish brown (10R 4/2-3/4) coarse siltstone nodules of average 175 cm in length (N=27) and 25 cm in thickness (N=10; Figure 7). Bounded on the bottom by rippled sandstone (average 10 mm in thickness (N=2)), the nodules, themselves, may be restricted to one upward-fining succession, or may envelope several upward-fining successions. From west to east, this interval becomes increasingly nodule-rich, finally resulting in a horizon that is completely altered. Cindy Looy (personal communication, 2003) has recovered pollen as well as a dispersed organic fraction from this

Figure 5. Plant remains in the Wagendrift Quarry section. (A) Small, isolated Glossopteris sp. leaf (at arrows) on bedding plane of siltstone-mudstone couplets (Facies 3). Leaf length 4 cm. (B) and (C) Isolated whorls of Phyllotheca sp. on bedding planes of siltstone-mudstone couplets. Scale = 10 m. (D) Deltoidospora directa, a simple deltoid, trilete spore associated with Mesozoic ferns. Scale = 10 m. (E) Weylandites lucifer, a nonsaccate pollen grain of Late Permian age. Scale = 10 m. (F) Lueckisporites virkkiae, a bisaccate taeniate pollen grain of Late Permian age. Scale = 10 m. (G) Horriditriletes ramosus, an acavate trilete spore. Scale = 10 m.

interval. Thin section analysis of the massive siltstone in which the nodules occur shows that the angular, quartz dominated fine to very fine sandstone (mean d=0.13 mm) gradationally fines into a coarse siltstone (mean d=0.05 mm). The clasts in the nodule are poorly sorted with the majority of the grains consisting of very fine sand (mean d=0.10 mm), but with larger (mean d=0.2 mm), angular, isotropic minerals scattered within the



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section, cm-scale successions consist of medium (mean d=0.35 mm) to very fine sandstone (mean d=0.10 mm). The rippled sandstone beds average 15 mm in thickness (N=2). The bottom and top contacts of each are sharp, and grain size coarsens upwards. Cindy Looy (personal communication, 2003) has recovered pollen in this facies.

Figure 6. Paleocurrent measurements at the quarry section. (A and B) are taken from ripple orientations in Facies 3, (C) is taken from the toes of balls in Facies 7, and (D) is taken from the ripple orientations in Facies 8. The multidirectional nature of ripples in these facies precludes an interdistributary bay interpretation.

Facies 7. Very Pale to Grayish Orange Very Fine Sandstone with Ball-and-Pillow Structures A thick, contorted bed of very pale to grayish orange (10YR 8/2-7/4) coarse siltstone to very fine sandstone exhibits ball-and-pillow and flame structures of various scales. This is overlain by a homogenous, massive bed of olive gray (5Y 3/2) coarse siltstone (Figure 7). Balland-pillow structures are typically 5 to10 cm high and 20 to 40 cm wide. Small (